A rotational motor is known having a stator, which configures a plurality of poles by assembling a plurality of core members so as to form a substantially circular shape.
In the above-described rotational motor, a width of the core member in a circumferential direction of the rotational motor is determined to be shorter at an outer side of the core member than at an inner side thereof in a radial direction of the rotational motor (in a radial direction of the circular-shaped stator) when seen from a rotational-shaft-extending direction. In other words, the core member is formed to be accommodated, with a minimum clearance, into a substantially trapezoid shape, whose width at an outer side thereof in the radial direction forms a longer side and whose width at an inner side thereof in the radial direction forms a shorter side being shorter than the longer side thereof. Consequently, the rotational motor is configured to be downsized and to output a high torque by minimizing a clearance generated between the adjacent core members.
A method for accommodating core members into a substantially trapezoid-shape with a minimum clearance is disclosed in JP2007-215364A (which will be referred to as reference 1 herein below). According to reference 1, a magnetic core portion itself is formed into the substantially trapezoid-shape and a coil is wound around a circumference of the magnetic core portion so as to achieve a substantially uniform thickness. A dimension of a cross section of the magnetic core portion is fixed in a magnetic-core-portion-extending direction. Further, a three dimensional shape is adapted in which a thickness thereof is larger at an inner side of the magnetic core portion than an outer side thereof in the radial direction in contrast with a difference in width between the outer side of the magnetic core portion and the inner side thereof in the radial direction, in order to maintain a uniformity of a magnetic flux density (see FIGS. 1 to 3 in reference 1, for example).
Further, a method for having a minimum clearance relative to a predetermined substantially trapezoid-shape is disclosed in reference 1. According to reference 1, a magnetic core portion, extending in the radial direction of the motor, is formed into a substantially fixed shape in every cross section thereof extending orthogonally relative to the magnetic-core-extending direction, so as to maintain a magnetic flux density. A coil is wound to a greater extent around an outer portion of the magnetic core portion than around an inner portion thereof in the radial direction (see FIG. 7 in reference 1, for example).
A method is disclosed in reference 1 by which the three-dimensional magnetic core portion, shown in FIGS. 1 to 3 of reference 1 and having variation in width and thickness thereof, is formed from a pressed powder material. However, the magnetic core portion made of the pressed powder material does not have a sufficient strength. Further, a method for forming the three-dimensional shape is disclosed in reference 1. An additional component is provided to the magnetic core portion, which is formed into the fixed shape in every cross section thereof and which is configured by stacking a plurality of magnetic steel plates conventionally adapted to a usual motor in layers. However, in such case, a manufacturing cost increases because the additional component is required.
On the other hand, the method for having the minimum clearance relative to the predetermined substantially trapezoid-shape is known in which the coil is wound around the magnetic core portion, which is formed into the fixed shape in every cross section thereof in the magnetic-core-portion-extending direction, so as to form a substantially trapezoid-shape. However, in such case, the wound coil is easily unwound. Specifically when a difference between a longer side and a shorter side of a trapezoid-shape is smaller relative to the core member disclosed in FIG. 7 in reference 1, in other words, in an electric rotational motor having a stator of a more number of poles, a wound coil is easily unwound.
An example of a case where a difference in width between a longer side A and a shorter side B of a trapezoid-shape is small is shown in FIGS. 7 and 8. In the example, a core member 105 is manufactured by a conventional winding method. The core member 105 is configured by winding a coil 108 around a magnetic core portion 150, which extends to be a fixed shape in every cross section thereof. The coil 108 is wound around the magnetic core portion 150 to form a plurality of layers. A winding number of an uppermost layer winding portion 181 is smaller than that of a second layer winding portion 182, which is provided at a lower side of the uppermost layer winding portion 181. (“Upper” and “lower” hereinbelow corresponds to a left side and a right side in FIG. 8, respectively.) Consequently, the core member 105 is accommodated in the trapezoid-shape shown by a double-dashed line in FIG. 7 with a minimum clearance.
A winding number of the uppermost layer winding portion 181 of the core member 105 increases when the difference in width between the longer side A and the shorter side B is small compared to when the difference in width between the longer side A and the shorter side B is large. Therefore, a length between a final winding portion 181a of the uppermost layer winding portion 181 of the coil 108 and an engagement portion 106 increases. Further, a cross angle α between a bridge portion 187, connecting the final winding portion 181a and the engagement portion 106, and a winding portion 181b, wound before the final winding portion 181a, increases.
When the cross angle α is equal to or larger than a predetermined value, the final winding portion 181a crosses over the winding portion 181b to be displaced from a predetermined position during a coil-winding process or a motor-assembling process. Further, as illustrated in FIG. 7, the bridge portion 187 is connected to the engagement portion 106, engaging with an end portion 189 of the coil 108, so as to extend obliquely. Therefore, operators and tools easily contact the bridge portion 187 during the motor-assembly process and the coil is easily unwound.
A need thus exits for an electric rotational motor which is not susceptible to the drawback mentioned above.